The disclosure relates to a projection apparatus for microlithography.
Furthermore, the disclosure relates to a projection exposure apparatus for microlithography.
Furthermore, the disclosure relates to a method for operating a projection exposure apparatus for microlithography.
Projection exposure apparatuses for microlithography generally include a light source, an illumination system, which processes the light beams emitted by the light source, an object to be projected, generally called a reticle or mask, a projection objective, called objective for short hereinafter, which images an object field onto an image field, and a further object, onto which projection is effected, generally called a wafer. The reticle or at least part of the reticle is situated in the object field and the wafer or at least part of the wafer is situated in the image field. The objective may define an optical axis with respect to which the optical elements associated with the objective are arranged. In general, said optical elements are rotationally symmetrical with respect to said optical axis and the optical axis is a normal to object field and image field. In this case, the design of the objective is called rotationally symmetrical.
If the reticle is situated completely in the region of the object field, and the wafer is exposed without a relative movement of wafer and image field, then the projection exposure apparatus is generally referred to as a wafer stepper. If only part of the reticle is situated in the region of the object field, and the wafer is exposed during a relative movement of wafer and image field, then the projection exposure apparatus is generally referred to as a wafer scanner.
During the exposure of the wafer, the projection exposure apparatus is operated with a predefined geometrical aperture and a setting predefined by the illumination system, for example a fully coherent, partly coherent, especially dipole or quadrupole setting. In this case, the geometrical aperture is understood to mean the quotient of numerical aperture and refractive index of the underlying medium. The geometrical aperture is thus identical to the sine of the half-side aperture angle of the objective. The geometrical aperture is predefined by the illumination system and/or defined by a diaphragm in the objective. Customary image-side geometrical apertures for objectives for microlithography are values of between 0.5 and 0.6, or 0.6 and 0.7, or 0.7 and 0.8, or 0.8 and 0.9, or else above the latter. A setting is generally predefined by optical elements of the illumination system such as e.g. an axicon, a diaphragm or a micromirror array or one or a plurality of changeable DOEs (diffractive optical elements).
During the exposure, from each field point associated with the object field, a maximum light beam trimmed by the aperture stop passes from the object field to the image field. In an ideally manufactured objective, the imaging aberrations of which are determined only by the optical design of the objective, the wavefront defined by said maximum light beam, in the vicinity of the image point associated with the field point, corresponds approximately to a spherical wave with the image point as midpoint. The possible resolution of such an objective is therefore determined by the diffraction orders which still lie within the geometrical aperture. Therefore, objectives of this type are also referred to as diffraction-limited objectives.
If the region between the last optical element of the objective and the wafer is filled with a gas as medium, then the refractive index thereof is generally approximately 1.00 and the geometrical aperture matches the numerical aperture.
If the region between the last optical element of the objective and the wafer is filled with a liquid as medium, then this is referred to as an immersion objective. One possible immersion liquid is water, which has a refractive index of approximately 1.43. The image-side geometrical apertures indicated above thus have to be increased by the factor of 1.43 in order to determine the assigned image-side numerical apertures. This results in image-side numerical apertures for immersion objectives of approximately 0.75 to 0.9 or 0.9 to 1.05 or 1.05 to 1.2 or 1.2 to 1.35 or else above the latter.
The possible resolution R, that can be achieved with such an objective for microlithography is inversely proportional to the numerical aperture NA and proportional to the operating wavelength λ of the objective and a process parameter k1:
      R    =                  k        1            ⁢              λ        NA              ,where k1 is always at least 0.25. The operating wavelength is generally 365 nm, 248 nm, 193 nm or 13 nm. In the case of 13 nm, the objectives are catoptric objectives, that is to say objectives consisting only of mirrors. These catoptric objectives may have an optical axis or not. These catoptric objectives are operated in a vacuum with geometrical—and correspondingly numerical—apertures of 0.2 to 0.25 or 0.25 to 0.3 or 0.3 to 0.4 or 0.4 to 0.45 or above the latter. Further types of objectives for microlithography are dioptric objectives, that is to say objectives consisting only of lenses, and also catadioptric objectives, that is to say objectives consisting of lenses and mirrors.
Diffraction-limited objectives and in particular objectives for microlithography react very sensitively to adjustment faults.
The term adjustment fault generally denotes a fault which arises as a result of an erroneous alignment of the optical elements of the objective with respect to one another or relative to the object and/or image field thereof.
In the context of this application, the term adjustment fault is intended to be interpreted more generally: it is intended also to encompass those faults which result from the materials used during production, assembly and subsequent operation of the objective. Besides the abovementioned erroneous alignments of the optical elements, the faults include variations in the refractive index—for short: index—of optically active materials, undesirable variations in the surface forms of the optical elements associated with the objective, drift in the relative position of optical elements in the mounts thereof, stresses during assembly of the objective with the resulting effects of stress birefringence and polarization-dependent index distributions thereby induced in the optical elements of the objective, and also heating of the objective with the resultant temporally variable, scalar index distributions, accompanied with alteration of the shape, of the optical elements associated with the objective. Finally, the changes in the optical elements of the objective or in the entire objective which arise under changing ambient influences such as ambient air pressure and ambient temperature, specifically ambient temperature of the objective, are also intended to be referred to as adjustment faults.
The individual image aberrations which determine the imaging performance of the objective have to satisfy specifications which ensure a sufficiently good imaging performance. The specifications concerning image aberrations are specified in general, as for example in the case of Zernike coefficients, by upper bounds for the absolute values of the image aberrations. As an alternative to the Zernike coefficients, it is also possible to use other coefficients resulting from function systems other than the Zernike polynomials.
As an alternative, besides the absolute values, use is also made of norms which functionally combine a plurality of image aberrations and which therefore have to satisfy a common specification.
Thus, by way of example, the rms values (root-mean-square) are specified as image aberrations using a, possibly weighted, Euclidean norm. Other norms are specifically coordinated with the design of the imaging optical assembly, in order, by way of example, to weight the field edge of the imaging optical assembly more highly than the field center thereof.
For a selection of field points in the image field of the objective and a given aperture, the wavefronts of the light beams associated with the field points are measured or calculated from measurement variables such as air pressure or temperature, for example, or temporally extrapolated on the basis of a prediction model from already known wavefronts and/or further measurement variables. The measurement of the wavefronts generally takes place interferometrically. The individual wavefronts, more precisely: their deviation from a spherical wave, are respectively expanded into a function system that is generally an orthogonal, in particular an orthonormal system. By way of example the Zernike polynomials form such an orthogonal system. The coefficients of this expansion, also called Zernike coefficients, are then referred to as image aberrations. Further image aberrations such as scale error, telecentricity error, overlay and depth of focus, best focus, and also image aberrations produced by integration of a plurality of field points, such as rms, grouped rms, residual rms, and fading, or other image aberrations, are derived on the basis of, in particular linear, models from the Zernike coefficients. Some of these image aberrations are defined in the context of the description or the figures.
As an alternative, some of these derived image aberrations can also be determined directly by measurements or prediction models. Combinations of measurements and prediction models are also employed. This is possible for example in the case of models of the image aberration prediction which come under the term model-based control. In that case, values such as, for example, air pressure and/or temperature, specifically ambient temperature of the objective, are used as parameters in a model for the image aberration prediction. These parameters are measured and the model is calibrated using the measured values. The image aberrations are subsequently predicted on the basis of the calibrated model. In this case, the parameters can be measured in a temporally periodic fashion. Prediction models for calculating unmeasurable image aberrations can be adjusted with directly measurable image aberrations as parameters for calibration. Predictions by the model and measurements can alternate: a prediction model is calibrated at predefined, preferably temporally equidistant, points in time by measurement of at least a selection of the image aberrations to be determined or other parameters from which the image aberrations to be determined can be determined. The determination of image aberrations is performed using one or a plurality of different prediction models between these points in time. For a more detailed explanation of model-based control, cf. Coleman Brosilow/Babu Joseph, Techniques of Model-Based Control, Prentice Hall International Series in Physical and Chemical Engineering Sciences, USA 2002.
With the increase in the resolution and, necessitated thereby, the reduction in the operating wavelength and/or increase in the numerical aperture, more stringent requirements are made of the imaging performance of the objective and smaller upper bounds are thus implicitly required for individual or plural image aberrations.
Furthermore, it cannot be assumed that a single adjustment in the area of manufacture of an objective will suffice to have the permissible image aberrations thereof in specification when the objective is first put into operation, since such an objective is generally not used at the place where it originates.
Furthermore, it cannot necessarily be assumed that a single adjustment of the objective will suffice to keep the latter in specification with regard to its permissible image aberrations over the lifetime of the objective.
Furthermore, it cannot necessarily be assumed that these specifications will be complied with during operation of the projection exposure apparatus. They can be violated even as early as during the transition from the exposure from one die to the subsequent die, or they can be violated even during the exposure of an individual die.
Thus, during operation of the projection exposure apparatus with light having the operating wavelength, alterations arise in the optical elements associated with the objective of the projection exposure apparatus, which alterations lead to, in part irreversible, changes in the optical properties of the objective. Compaction, rarefaction and chemically governed alterations of possible coatings of the optical elements shall be mentioned here by way of example. Further, irreversible alterations are produced by drifts of optical elements in the mounts thereof, said drifts being established with increasing time. Other alterations are reversible in their nature, such as e.g. lens heating with the alteration of form implied thereby and with the alteration of the distribution of the refractive index of the lens. These lead to time-dependent alterations of the optical properties of the objective.
Therefore, objectives for microlithography have been supplemented in the course of their development by an increasing number of manipulation possibilities. The latter can be used to counteract the changes in the optical properties of the objective in a controlled manner. Use is made of manipulators which shift, rotate, exchange, deform, heat or cool one or a plurality of optical elements associated with the objective, such as lenses, mirrors or diffractive optical elements. In particular, aspherized plane plates are provided as exchange elements in the objective. Exchange elements can also be optical elements of an objective which are provided with manipulators. These elements are preferably some of the first and last optical elements of the objective as seen in the direction of light propagation, or some of the optical elements situated in the vicinity of an intermediate image of the objective, or some of the optical elements situated in the vicinity of a pupil plane of the objective. The term vicinity is defined here with the aid of the so-called paraxial subaperture ratio. In this respect, cf. WO2008034636A2, for example; in particular pages 41 and 42 therein, which contain the definition of the subaperture ratio, shall be incorporated within their full scope in this application. An optical element is referred to as in vicinity of a pupil plane or near a pupil plane if the absolute value of its paraxial subaperture ratio is close to 1; by way of example, all optical elements which have a paraxial subaperture ratio of greater than 0.8 should be referred to as near the pupil. Correspondingly, all optical elements which have a paraxial subaperture ratio with an absolute value of less than 0.2 should be referred to as near the field or near an (intermediate) image or equivalently in vicinity of an (intermediate) image plane. The term (intermediate) image is equivalently named as field.
Thus, by way of example, WO2008037496A2 discloses an objective for microlithography containing an optical element to which a multiplicity of forces and/or torques are applied by a manipulator, such that said optical element attains a local variability with regard to its form.
Thus, by way of example, WO2008034636A2 or WO2009026970A1 discloses a plane plate in an objective for microlithography. Conductor tracks to which current can be applied are situated in or on said plane plate. In the case of the change in temperature caused as a result, the refractive index of the plane plate can be influenced locally, such that the plane plate has a local variability with regard to its refractive index.
Thus, by way of example, EP851305B1 discloses a pair of plane plates, so-called Alvarez plates, in an objective for microlithography. This pair of Alvarez plates has an asphere in each case on the mutually facing surfaces of the plates, said aspheres compensating for one another in terms of their optical effect in a predetermined relative positioning of the plates with respect to one another. If one or both of the plates is or are moved perpendicularly to the optical axis of the objective, then the optical effect of these Alvarez plates is established.
Thus, for example, EP1670041A1 discloses an apparatus which serves for the compensation of image aberrations that are introduced into the objective for microlithography specifically as a result of the absorption of dipole illumination. An optical element situated in a pupil plane of the objective experiences non-rotationally symmetrical heating in the case of dipole illumination. The optical element has applied to it additional light from a second light source, which emits light preferably having a different wavelength than the operating wavelength, at least approximately complementarily to said heating. Undesired image aberrations are thereby compensated for or at least reduced.
Manipulators that deform an optical element are distinguished by their particularly rapid response behavior. A general introduction to rapidly responding manipulators from the field of telescope technology is given in R. K. Tyson: Principles of Adaptive Optics, Academic Press, Inc., ISBN 0.12.705900-8.
Every manipulator has a certain number of degrees of freedom. This number of degrees of freedom can vary very greatly. Thus, by way of example, a manipulator which displaces a lens in a predefined direction has precisely one degree of freedom. By contrast, a manipulator which includes electrical conductor tracks which apply heat to a lens has degrees of freedom in a manner corresponding to the number of conductor tracks to which voltage can differently be applied.
In the context of this application, the term adjustment is understood to mean not just the alteration of the spatial arrangement of the optical elements of the objective with respect to one another, but any manipulation of the objective using the manipulators listed above.
In addition to the statements made above, the term adjustment is understood below to mean three subforms:                the initial adjustment during the assembly of the objective,        the repair adjustment, requiring an interruption in the operation of the projection exposure apparatus, and        the fine adjustment during the operation of the projection exposure apparatus.        
The adjustment which is necessary, under certain circumstances, before the first use of the projection exposure apparatus at the site of use is intended likewise to come under the term of repair adjustment.
The fine adjustment takes place, inter alia, for the correction of image aberrations which arise on account of the heating of the objective. In the case of the fine adjustment, adjustment in real time is also an expression employed.
The following time periods for carrying out the fine adjustment, that is to say for determining the image aberrations, calculating the movement distances of the manipulators—this calculation referred to hereinafter as “solving the inverse problem”—and moving the manipulators, are understood depending on intended use, throughput and type of the objective and the available manipulators thereof: up to 30000 ms (milliseconds), or up to 15000 ms (long-time behavior), or up to 5000 ms, or up to 1000 ms, or up to 200 ms, or up to 20 ms, or up to 5 ms or up to 1 ms (short-time behavior).
The fine adjustment is subdivided in particular into the three subsections                determining the image aberrations        solving the inverse problem        moving the manipulators        
For these three subsections, the following time periods for carrying them out are estimated relative to one another: determining the image aberrations and solving the inverse problem relative to moving the manipulators 50% to 50% and determining the image aberrations relative to solving the inverse problem 60% to 40%. Therefore, up to 6000 ms, or up to 3000 ms, or up to 1000 ms, or up to 200 ms, or up to 40 ms, or up to 4 ms, or up to 1 ms, or up to 0.2 ms, are generally available for solving the inverse problem during the fine adjustment.